1. Field of the Invention
The present invention relates generally to methods of packaging and sterilizing elastomeric articles in a manner that prevents and/or reduces degradation to the articles, particularly degradation that may be caused and/or accelerated by sterilization techniques such as gamma irradiation, x-ray irradiation, and electron-beam processing. In certain aspects of the invention, packaged elastomeric articles containing one or more antidegradants, such as antioxidant and/or antiozonant compounds, are also provided. The methods of providing degradation-resistant elastomeric articles in accordance with the present invention may also be used to reduce the occurrence of cracking and discoloration in elastomeric articles, regardless of whether they are subjected to sterilization.
2. Description of Related Art
Although the technology involved in the production of synthetic polyisoprene (PI) latex has been used for a long period of time, synthetic PI latex has only been used commercially for glove manufacturing for about the last decade. This was in part because the price of synthetic PI latex is significantly higher than natural rubber latex, although both latices have polyisoprene as their active ingredients.
However, in view of the increased level of awareness regarding allergies to proteins present in natural rubber latex, there has been a shift towards the use of synthetic latices that do not contain natural rubber latex proteins, especially for use in making medical devices that come into contact with the skin. Taking cost and performance into consideration, synthetic latices that are suitable for glove manufacture include nitrile latex and polyurethane latex for examination gloves, and polychloroprene latex and PI latex for surgical gloves. For surgical gloves, PI latex is preferred over polychloroprene, even though it is more expensive, because it provides the gloves with properties that mimic those of natural rubber, particularly tensile strength, ultimate elongation, softness and comfortable feel.
While the synthetic PI polymer is chemically similar to the PI polymer in natural rubber, there are some differences in the chemical structure of the polymer. In addition, there are also differences between the compositions of synthetic PI latex and natural rubber latex. Depending on the catalyst used for preparing the PI, synthetic PI contains about 90 to 98.5% cis-polyisoprene, 1 to 5% trans-polyisoprene and 0.5 to 5% other forms of polyisoprene. The PI in natural rubber comprises about 98% cis-polyisoprene and 2% trans-polyisoprene. In terms of overall composition, natural rubber latex comprises about 94% PI latex particles and about 6% non-rubber materials, whereas synthetic PI latex comprises about 97-99% PI latex particles and about 1-3% colloidal stabilizers. The colloidal stabilizers, such as surfactants or carboxylic acid soaps, help to keep the dispersed PI particles stable in the aqueous phase. The non-rubber materials in natural rubber comprise proteins, lipids, fatty acid soaps, etc. These non-rubber materials play an important role in the vulcanization of rubber in the latex and some are believed to have antioxidant properties.
Due to these differences between synthetic PI latex and natural rubber latex, in terms of both the compositions of the PI polymers and the overall compositions, the vulcanization formulation for crosslinking the PI polymers in the latices is different for each of the two types of PI latex. Since commercial synthetic PI latex is relatively new as compared to natural rubber latex, the amount of published information on the compounding formulations for synthetic PI latex is limited. A brief review is provided below.
Generally, the compounding formulation for vulcanization of rubber comprises the following classes of materials (a) crosslinking agents (usually sulfur or sulfur donors), (b) vulcanization accelerators, (c) vulcanization activators, and (d) antidegradants. For reference, the following abbreviations are used in this application: ZDEC—Zinc diethyldithiocarbamate, ZDBC—Zinc dibutyldithiocarbamate, ZDNC—Zinc diisononyldithiocarbamate, ZDBeC—Zinc dibenzyldithiocarbamate, TMTD—Tetraethyl thiuram disulfide, TETD—Tetraethyl thiuram disulfide, TBeTD—Tetrabenzyl thiuram disulfide, MBT—2-mercaptobenzothiozole, ZMBT—Zinc 2-mercaptobenzothiozole, DPTU—Diphenyl thiourea, DPG—Diphenyl guanidine, DIXP—diisopropylxanthogen polysulfide, DIX—Diisononyl xanthogen, XS—Xanthogen sulfide, Wingstay L—butylated reaction product of p-cresol and dicyclopentadiene, Aquanox L—aqueous dispersion of butylated reaction product of p-cresol and dicyclopentadiene, AO2246—2,2′-methylene-bis-(4-methyl-6-t-butylphenol), AO264—2,6-di-tert-butyl-4-methylphenol, and MMBI—4- and 5-methyl-2-mercapto-benzimidazole.
Henderson (International Latex Conference 2000, Akron, Ohio) disclosed a formulation using sulfur, three accelerators (ZDEC, ZMBT and DPG, each at a fixed level), zinc oxide, and an antioxidant (Wingstay L).
Wang et al. (U.S. Pat. No. 6,828,387) disclosed formulations using sulfur, three accelerators (ZDEC, ZMBT and DPG, at different ratios), zinc oxide, and an antioxidant (Wingstay L).
Sak et al. (U.S. Pat. No. 6,618,861) disclosed a formulation using sulfur and a sulfur donor (TMTD), four accelerators (ZMBT, ZDEC, ZDBC and DPTU, each at a fixed level), zinc oxide, and two phenolic type antioxidants (AO02246 and AO0264).
Chakraborty et al. (2nd International Rubber Glove Conference 2004, Kuala Lumpur, Malaysia) disclosed formulations using sulfur, two combinations of two accelerators (ZDNC and DIXP, or ZDEC and MBT), zinc oxide, and two antioxidants (AO2246 and MMBI).
Webster et al. (International Latex Conference 2004, Akron, Ohio) disclosed formulations using sulfur, undisclosed accelerator systems, zinc oxide, and an undisclosed antioxidant.
Jole Van (WO 2007/017375) disclosed formulations using sulfur, two accelerators (ZDEC and DPG), zinc oxide, and an antioxidant (Aquanox L). Jole Van (WO 2007/017368) also disclosed formulations using sulfur, accelerators (DIXP, and alkyl dithiocarbamates of various chain lengths, such as ZDNC, and DPG), zinc oxide, and an antioxidant (Aquanox L).
Lucas (WO 2003/072340) disclosed formulations using sulfur, accelerators (various combinations comprising DIXP, DIX, XS, TETD, TBeTD, and ZDBeC), zinc oxide, and an antioxidant (Wingstay L).
Teoh et al. (U.S. Pat. No. 7,179,415) disclosed a neoprene article formed using sulfur, zinc oxide, and accelerators (Rhenocure, DPG, and ZDBC), from 1.0 to 3.0 phr of an antioxidant (e.g., Wingstay L), and from 0.5 to 2.0 phr of an anti-tack agent (e.g., Michem Lube-180).
Bourne et al. (U.S. Pat. No. 6,195,805) disclosed a neoprene article formed using a vulcanizer, an activator, an accelerator, from 0.25 to 5.0 phr of an antiozonant, and from 0.1 to 3.0 phr of an antioxidant. The neoprene articles were packaged in paper and then sterilized using gamma irradiation or electron beam sterilization.
Weikel et al. (U.S. Pat. No. 6,306,514) disclosed elastomeric flexible articles having a lubricant composition provided on the skin-contacting layer, in order to improve lubricity. The elastomeric base layer may be formed from a synthetic rubber latex emulsion that includes a sulfur or sulfur-containing vulcanizer, a zinc oxide activator, a dithiocarbamate accelerator, a phenol-type antioxidant, and an emulsified wax as an antiozonant.
Accordingly, there is a need in the art for methods of packaging and sterilizing elastomeric articles in a manner that prevents and/or minimizes degradation to the articles, particularly degradation that may be caused and/or accelerated by sterilization techniques such as gamma irradiation, x-ray irradiation, and electron-beam processing. There is also a need for packaged elastomeric articles produced according to the methods of the invention. Such packaged elastomeric articles exhibit improved degradation-resistance as compared to elastomeric articles that are not packaged in accordance with the methods of the present invention. The packaged elastomeric articles beneficially exhibit a reduced incidence of cracking and discoloration, regardless of whether they are subjected to sterilization techniques.